In many clinical situations, a lumen in a human or animal body, such as a blood vessel or a urinary tract, can require internal support to ensure proper fluid flow. Support for a lumen can be provided by implantation of a stent in the lumen, which can maintain the radial integrity of the lumen.
A stent is typically a tubular metallic or polymeric body, which is carried on a dilatation catheter to a specific vascular location. The implantation of a stent to maintain patency of a body lumen is well known in the art. In one cardiovascular application, a stent is mounted on a balloon catheter and positioned at the appropriate site within an artery. The balloon is dilated to expand the stent against the vascular wall. The balloon is thereafter deflated and removed, leaving the expanded stent in place in the artery. Due to the structural integrity of the stent, the arterial wall is supported by the stent and prevented from recollapsing. An example of this method is described in U.S. Pat. No. 5,292,321 which is herein incorporated by reference. The stent may also be expandable via thermal energy through the use of electrical activation or radio-frequency electromagnetic irradiation, as described in U.S. Pat. No. 5,562,641.
Several difficulties are associated with the positioning and use of the above-identified stents. For example, it is difficult to simultaneously maintain the radial rigidity and the longitudinal flexibility of the stent to facilitate proper stent delivery. Additionally, the rapid expansion of the stent by such methods can injure the lumen to be protected, often leading to restenosis in the vessel lumen. Ideally, a stent would be expanded slowly over a prolonged period of time to reduce the likelihood of such injury and resulting restenosis.
Furthermore, it has been discovered that over time the movement, activity and changing medical condition of a patient can adversely affect stent operation and/or dictate that a previously expanded stent be further expanded to continue and/or augment the beneficial support provided by the stent. Currently used stents are not capable of prolonged expansion since they are not expandable in vivo, after removal of the positioning catheter; similarly, stent expansion after removal of the positioning catheter cannot be accomplished without re-entry into the lumen.
Accordingly, there remains a need in the art for a stent that can be remotely expanded or contracted in vivo over a prolonged period of time.